A variable-frequency multi-connected heat pump system is a complex refrigerating system with a complex structure, a large architecture, highly coupled internal parameters, and a variety of boundary conditions, which is characterized by a highly varying incumbent load demand, a large number of connected indoor units, a varying complex operating condition, etc., and is an important direction toward which air conditioners are evolving. As illustrated in FIG. 1, the multi-connected heat pump system generally includes one or more outdoor units 01, one or more indoor units 02, a central control network (CS-NET) 03, a refrigerant pipe 04, a manifold pipe 05, and a communication line 06, where the outdoor units constitute a set of outdoor units controlled by the central control network through the communication line, and the outdoor units are connected with the indoor units through the refrigerant pipe and the manifold pipe. As compared with a number of air conditioners at home, the outdoor units of the multi-connected heat pump system are shared so that the cost of the devices can be lowered effectively, and the respective indoor units can be centrally managed, where one or more of the indoor units can be started separately or concurrently into operation, thus making it more flexible to control.
At present, people are demanding high comfort of their living. In an engineering application, there are some of the indoor units in the multi-connected heat pump system that are refrigerating while the other indoor units are heating. In order to make full use of advantageous energy-saving in the multi-connected heat pump system, heat in the system is recovered in the prior art to enable the operating indoor units to refrigerate and heat concurrently. However, generally a number of four-way valves have to be arranged to allocate the refrigerant and to recover discharged heat, thus making it complex and costly to control the pipes of the system. Moreover, if the prior art is applied in a rainy season and a moist region, then the system has to refrigerate for the purpose of temperature lowering and dehumidification, and if the temperature of the environment is low, then the effect of dehumidification may be poor, and the temperature of outgoing air may be low, thus resulting in degraded thermal comfort.